1. Field of the Invention
The present invention relates to the field of digital signal processing and, more specifically, to a method and apparatus for broadcasting, and synchronizing a VCO to, a serial digital signal.
2. Background Art
Electronic communication of serial digital data is straightforward if the sending clock frequency and phase are known by the receiver, but problems arise when the transmitted data clock frequency and phase are unknown. In such situations, the exact phase of the sending clock can be inferred by the temporal positioning of the received data bits, a technique that is currently in practice. In such a scheme, a receive voltage controlled oscillator is affected by a phase comparator, detecting the phase of the receive clock on the receipt of each data bit. The result of this phase comparison can be applied to the voltage control input of the VCO to achieve the locking of phase of the receiver to the incoming data. This technique, however, is only useful when the transmitter clock is within the capture range of the receiver VCO/phase locking loop. The receiver must have prior knowledge of the expected incoming data rate for effective phase locking to occur.
The transmitted data rate may vary from transmission to transmission, provided some means is provided to inform the receiving station as to the approximate sending data rate. Typically, pilot tones precede the data message, indicating the following data rate, and can be used by the receiving station to set its receive clock to an approximate frequency, whereby subsequent phase locking can occur once the data transmission begins.
Although a parallel bus is ideal for interconnecting a distributed multi-channel system, for a point-to-point connection, the use of fiber optics is desirable, particularly as distances increase. An optical fiber is a filament (e.g., glass or plastic) that is formed in such a way that light is constrained to travel along it. Signal transmission is achieved by modulating the power of a light-emitting diode (LED) or small laser coupled to the fiber. A phototransistor at the end opposite the LED converts the received light back to an electrical signal for eventual signal processing.
Optical fibers have numerous advantages over electrical cabling. Optical fibers neither generate, nor are prone to, electromagnetic interference and, as they are insulators, ground loops cannot occur. On the other hand, electrical cabling may generate undesirable radio frequency signals.
Many fiber optic systems are currently available. One fiber optic system, manufactured by Sharp Corporation, consists of a transmitter and a receiver that both operate from of a 5 volt power supply, and provide connection through logic levels. The transmitter works by turning on an LED when the logic input is high, and the receiver provides a logical high output when sufficient light is received at the other end of the interconnecting cable.
Since the receiver must pick up and amplify the LED signal from a photodiode, very high gains must be used, and the signals must be AC coupled. As a result, the output of the receiver is a replica of the logic level at the transmitter, but the LED must be alternating on and off at a high rate for the receiver to function properly. Minimum signal rate requirements of 100 kHz are not uncommon.
Further, Tdlh (delay time from transmitter turning on to receiver output going high) is different from Tdlh (delay time from transmitter turning off to receiver output going low). Assume that 8 channels each consisting of 24-bits of digital information are transmitted over a fiber optic cable. These 192 bits of information cannot simply be set end to end and sent to the transmitter, since if all of the bits were zero, the LED would not alternate on and off, causing the receiver to malfunction. Also, if a long string of bits has the same polarity, the receive logic has no way of knowing how many bits of that same polarity have passed. To accurately convey data, then, some clock information (regular changes in the data pattern) must exist in the data stream to give clues to the receive logic as to the data transmission rate.
In the following discussion, Non Return to Zero Inverted (NRZI) encoding is understood to apply. In NRZI, transmitted data is represented by a transition within the channel from one binary condition to the other, and such a transition is noted as a transmitted `1`. Periods of time without transitions, however, may or may not equivalently represent the data sequence to be transmitted. The data to be transmitted, a sequence of data 1's and 0's, is transformed via a modulation code, prior to transmission. At the receive end, the sequence of received transitions, is decoded into the original data 1 and 0 sequence by a receive decoder.
The main purpose for modulation coding is to provide adequate transmit clock information. An adequate modulation coding scheme is one that allows a continuous stream of data zeros or ones to be represented by a channel pattern that contains enough transitions (channel 1's) that the receive clock can accurately infer the transmit clock's phase.
Several techniques for receiving standardized modulation codes are available. For example, FM coding is commonly used to convey digital data. In the FM modulation scheme, two transmit clock cycles are used to represent each serially transmitted data bit. A data `1` is represented by two channel transitions (two channel `1`s), and a data `0` is represented by a single channel transition (single channel `1`).
In the case of FM coding, the receiver can deduce the correct receive clock frequency by the receipt of a single data `1`, which is represented by two transitions, one during each transmit clock period. However, if the data message happens to contain only data zeros, the receiver, not knowing the transmit clock frequency, will not be capable of knowing whether the message was correctly a stream of data `0`s, or incorrectly, a stream (half as long) of `1`s, at half the original clock frequency.
FM could be used to unambiguously communicate a message, and achieve clock synchronization as well, provided the data was previously grouped into blocks of predefined length, and sufficient extra bits were used to define the clock frequency, such as a long string of 0's appended to each block, and a unique synchronization pattern was added to denote the beginning of each data block. Such a synchronization pattern could consist of a period of two or more clocks without a channel transition, which would normally violate the FM encoding rules for real data.
Such a system would suffer from extreme inefficiency, as more than two clock periods would be required to communicate each data bit, leading to a wide required channel bandwidth. The presence of bit jitter on the received transition would make the accurate determination of the correct receive clock frequency very difficult. Such bit jitter problems can be reduced by increasing the number of appended data 0's to the message, to more accurately define the correct receive clock frequency, but this leads to yet further reductions in the scheme's efficiency, in terms of data transmitted versus required channel bandwidth.
The SDIF-2 (Sony Digital Interface Format), the PD (ProDigi) format, and the AES/EBU interface all allow transmission of audio digital data from recorder to recorder. SDIF-2 and PD formats do not include clock information in the data signal, and require a separate connection between devices to accomplish synchronization. Although the AES/EBU interface is self-clocking and self-synchronizing through a single serial interface, it is designed to transmit only 2 channels at a bit rate of 3.072 MHz and a sample rate of 48 kHz. Additionally, the AES/EBU interface uses FM channel code, which has a high overhead (50%) and is designed to transmit over a single twisted wire pair.
It is desirable to be able to send more than two channels of digital audio information between two or more devices without having to provide a separate synchronization channel or connection. Further, it is desirable to have a self-clocking, self-synchronizing interface format with high data efficiency, able to synchronize over a wide range of sampling rates.